Emissions regulations in the United States, Europe, Japan, China and other countries are becoming increasingly stringent in terms of the emissions levels that are permitted for diesel engines. For example, the U.S. regulations limit, among other things, the levels of particular matter and oxides of nitrogen (commonly referred to a NOx) that may be emitted from a diesel engine. In addition to the regulations governing diesel engine emissions, the U.S. government has also promulgated regulations requiring the sulfur content of highway diesel fuel to be below a certain level (e.g., 15 ppm). This too has been done in an effort to facilitate the reduction of the particulate matter emitted by diesel engines.
Although reducing the sulfur content of diesel fuel helps to reduce undesirable emissions, this often has the effect of reducing the lubrication levels of the fuel. The processing steps that are used to produce the standard U.S. low sulfur diesel fuel, or ultra low sulfur diesel fuel as it is often called, generally result in a reduction in the average normal carbon chain length, which also tends to reduce the lubrication levels of the fuel. Fuel blenders sometimes compensate for the reduced lubricity, at least in part, through the use of additive packages, but this generally does not result in the desired lubrication levels. Other specialty fuels, such as Toyu and JP8, also feature shorter than average normal carbon chain lengths and lower sulfur than traditional U.S. 2D diesel fuel, and therefore also possess relatively low lubrication levels.
One area in which the reduced lubricity of diesel fuel has a significant impact is the fuel system of diesel engines, particularly the pumps and injectors of the fuel system. Pumps and injectors include key parts that move or reciprocate relative to other parts millions of times during the life of an engine. When the fuel serves as a lubricant to these parts, which is often the case, a reduction in the lubricity of the fuel can significantly increase the rate of wear in these parts, which in turn leads to earlier failure of the parts and/or the entire fuel system. For example, conventional inline plunger or piston fuel pumps that are used to generate the high fuel pressure in common rail fuel systems may include control valve assemblies that actuate millions of times during the life of the pump. Although these control valve assemblies may experience little wear over time when used with pump traditional 2D diesel fuel, their use with newer diesel fuel formulations that have reduced lubricity may cause these control valve assemblies to prematurely fail due to the increase in wear experienced by the control valve assemblies when used with these newer fuels.
Although certain materials may be selected that would exhibit a resistance to wear in the presence of a fluid with low lubricity levels, the use of these materials in a fuel systems application is often very difficult. For example, a ceramic material may provide acceptable resistance to wear in the presence of a fuel with low lubrication levels. However, the incorporation of a ceramic material into a fuel system is made difficult due to the fact that ceramics tend to be very hard, making the manufacture of ceramic parts more difficult, they tend to be expensive, making extensive use of the material cost prohibitive, and their brittle nature makes them susceptible to failure when subjected to tensile stresses, which are difficult to avoid in fuel systems applications. Also making the selection of an appropriate material difficult is the corrosive nature of many diesel fuels. Materials that would otherwise possess favorable characteristics may not be suitable for use in a fuel system because of their susceptibility to corrosive attack by the diesel fuel.
Various efforts have been made to address wear issues in high-pressure pumps. One example of such an effort is described in U.S. Pat. No. 6,019,125, issued Feb. 1, 2000 (“the '125 patent”). The '125 patent discloses a valve that fits within a cylindrical cavity formed in the pump body and that is retained in position by an overlying retention plug. The valve includes a cage-shaped valve body that includes an upper part and a lower part. The upper part includes four ribs, while the lower part includes a valve seat. The valve also includes a valving element located within the valve body that is guided by the four ribs and that is maintained in the closure position by a spring. Although the valve disclosed in the '125 patent appears to have been designed with wear in mind, it appears to have been designed not to minimize wear but rather to be easily replaceable after excessive wear occurs. Moreover, the '125 patent fails to appreciate the different wear characteristics that may result from the use of different fuel compositions, such as the different wear characteristics that may result from the use of traditional U.S. 2D diesel fuel versus the new U.S. ultra low sulfur diesel fuel.
It would be advantageous to provide a relatively simple, reliable, durable, and inexpensive control valve assembly that could effectively operate in a fuel system in which low lubricity diesel fuels are used.